Why Does Go Throw a Runtime Error When a Slice\'s Length Exceeds its Capacity?

Understanding Slices: Capacity vs. Length

When dealing with slices in Go, it's crucial to comprehend the relationship between capacity and length. Capacity refers to the underlying array's size upon which the slice operates, while length specifies the number of elements currently included within the slice.

Runtime Error: Slice Length Exceeds Capacity

The error "runtime error: makeslice: cap out of range" occurs when attempting to create a slice with a capacity less than its length. This error arises because slices by design maintain an invariant where the length can never exceed the capacity:

0 ≤ len(s) ≤ cap(s)

In your example code:

type b []int
var k = make([]b, 10, 5)
fmt.Println(k[8])

You have defined a slice k of type []b, where b is another slice type. However, you have attempted to create this slice with a capacity of 5, which is insufficient to accommodate its length of 10. Hence, the runtime errorが発生します。

Why Not a Compile-Time Error?

In certain cases, like yours where the values for capacity and length are static, a compiler could potentially detect the error at compile-time. However, this is not always feasible. Consider the following code:

package main

import (
    "fmt"
    "rand"
)

func main() {
    k := make([]int, rand.Int(), rand.Int())
    fmt.Println(k)
}

Here, the values for capacity and length are determined dynamically at runtime using the rand package. As such, compilers are unable to statically verify that the capacity will always exceed the length and, therefore, must delegate this check to runtime.

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